Superposition model analysis of nickel(II) ions in trigonal bipyramidal complexes exhibiting huge zero field splitting (aka ‘giant magnetic anisotropy’)

Abstract

Potential single-ion magnet Ni2+ systems: [Ni(Me6tren)Cl](ClO4) and [Ni(Me6tren)Br](Br) reveal unusually high zero field splitting (ZFS). The ZFS parameter (ZFSP) Dexpt=−120 to −180cm−1 was determined indirectly by high-magnetic field, high-frequency electron magnetic resonance (HMF-EMR). Modeling ZFSPs using the density functional theory (DFT) codes predicts D values: −100 to −200cm−1. Such ZFSP values may seem controversial in view of the D values usually not exceeding several tens of cm−1 for Ni2+ ions. To corroborate or otherwise these results and elucidate the origin of the huge ZFS (named inappropriately as ‘giant uniaxial magnetic anisotropy’) and respective wavefunctions, we have undertaken semiempirical modeling based on the crystal field (CF) and spin Hamiltonians (SH) theory. In this paper, a feasibility study is carried out to ascertain if superposition model (SPM) calculations may yield such huge D values for these Ni2+ systems. Using crystal structure data for [Ni(Me6tren)Cl](ClO4) and [Ni(Me6tren)Br](Br) as well as taking into account the Jahn-Teller distortions of five-fold coordinated Ni-complexes revealed by DFT geometry optimization, the ZFSPs are predicted for several structural models and wide ranges of model parameters. The results indicate that in the presence of the Jahn-Teller distortions, the SPM-predicted ZFSP D may achieve the observed magnitudes, whereas the positive as well as negative D-signs are obtainable. Further studies based on SPM calculations of CF parameters and diagonalization of the Hamiltonian (Hfree ion+HCF) will allow considering the wavefunctions compositions and thus actual origin of the splitting between the ground and first excited state.